CONFERENCE REPORT I ADVANCES IN PHOTOVOLTAICS
manipulation using quantum confinement in silicon dots having diameters of just 2-5 nm. Another goal for this team is the optimisation of epitaxial germanium-on-silicon substrates for stacked junction growth. And they are also working on the development of hot carrier cells, in which slower thermal relaxation at the band edge is controlled by manipulation of the lattice’s phononic properties, again using quantum dots.
Thinning silicon to cut costs Cigang Xu from Oxford Instruments talked about the trend for reducing the quantity of silicon in devices by simply thinning wafers to 100-150 µm whilst maintaining their thermal and ultraviolet stability and passivation properties. He described the progress made in a European-funded project focused on developing aluminium oxide deposition for the rear side of cells using plasma-enhanced CVD (a technique that offers clear routes to industrial scale-up). This process has great potential for precise growth control: Precursor supply (trimethlyl aluminium and nitrous oxide), RF power, pressure, flow rates, and temperature are all available parameters that can be used for optimisation, whilst monitoring plasma conditions. According to Xu, ultimately it’s necessary to find an appropriate balance between growth rates, precursor usage, uniformity, and film properties.
Transfer of a research project in thin-film devices at Colorado State University to the production line were described by Kurt Barth of Abound Solar. The result is a rugged CdTe PV module that is now being fabricated on a commercial scale. A first- generation tool was built in 2009, and just 19 months later the first modules were coming off the production line. Barth revealed that manufacturability has been a key driver at all stages of development. He said that the resultant product is robust, utilising a unique encapsulation design with an edge seal enclosing internal dessicators. This results in impressive long- term performance, with tests revealing that devices maintained high performance at 85 °C and 85 percent humidity for well in excess of the 1000 hours required for certification.
At Abound automated cell manufacture takes about two-and-a- half hours from start to finish. Glass is loaded and a laser scribes grooves into this substrate, before a proprietary method based on thermal sublimation deposits a CdTe-based film. Metal sputtering to add back contacts follows, and finally the cell is encapsulated.
According to Barth, Abound will be producing 70,000 modules per month by Fall 2012. Production yield already exceeds 80 percent and total area efficiency is up to 10 percent – further improvements are expected through fine tuning of thickness and processing. The Colorado-based CdTe outfit also has plans to ramp its manufacturing capacity. Construction of a second production line with 65 MW capacity is underway and a third is planned that should lead to a 170 MW capacity by 2012. A new facility with 10 lines is also in the pipeline, and Barth says this should enable Abound to expand its manufacturing capacity to 1500 MW by 2014.
Another speaker talking about commercial drivers for cost reduction and improved capability was Stuart Irvine, who is the
CdTe-solar cell manufacturer Abound Solar won a 1.5 MW contract in Polpenazze Del Garda,Italy
Director of the Centre for Solar energy Research at Optic Technium and an academic at Glyndwr University. According to Irvine, although thin-film technology actually offers only a slight cost reduction compared to silicon at this stage, it has great potential to become far cheaper. That’s because thin-film technology is relatively new, so it is only now growing in maturity, with high-volume manufacture leading to economies of scale. Irvine argued that it’s still a close race between these PV rivals, and he predicted that thin-film technology would represent about a quarter of the PV market by 2013.
Irvine went on to point out some of the materials challenges for thin-film PV: An increased conversion efficiency; reductions in the cost and amount of semiconductor material used; and the development of cheaper, lower-energy processing methods, with high throughput. He also pointed out that there are issues of durability and product lifetime. One option for trimming costs is to use ultra-thin films. However, models of the costing breakdown that have been produced by the UK PV Supergen consortium illustrate that the relatively small materials component of total production costs doesn’t justify this switch right now. The situation could change, however, given the uncertainty surrounding future materials prices – tellurium prices are critical.
Another topic discussed by Irvine was CdS deposition on windows by atmospheric pressure MOCVD at growth temperatures of 200-450 °C. This involved the addition of zinc to increase transmission. He and his co-workers are also looking at device efficiency at various film thicknesses and trying to determine how thin a film can be whilst maintaining its properties. It’s not simply absorption that must be considered, explained Irvine – there are other effects to take into account in cells that are just a few microns across. To thoroughly investigate this topic, the team have turned to micro-light beam induced current mapping, a technique that can nicely illustrate issues relating to uniformity and pin-holes, with different wavelengths highlighting different effects. Irvine summarised current work for the team, including enhancing absorption with light capture through scattering, optimisation of back reflectors
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